[FFmpeg-devel] AAC decoder round 9

[Robert Swain]

2008/8/20 Michael Niedermayer <michaelni at gmx.at>:
> On Wed, Aug 20, 2008 at 07:44:24PM +0100, Robert Swain wrote:
>> 2008/8/20 Robert Swain <robert.swain at gmail.com>:
>> > 2008/8/20 Vitor Sessak <vitor1001 at gmail.com>:
>> >> Robert Swain wrote:
> [...]
>> >>> It seems the various conditions on in[] and f0 cause it to exit with
>> >>> an error with the coef[] vectors I've tried and as they're potentially
>> >>> valid, that's disconcerting for using this code.
>> >>>
>> >>> compute_lpc_coefs() accepts a two dimensional lpc array as an
>> >>> argument. I'm guessing this so that the coefficients are available for
>> >>> various orders ready for testing to choose which is best or something
>> >>> like that.
>> >>>
>> >>> Do you have any advice for how to proceed? I'm going to keep prodding
>> >>> eval_lpc_coeffs() in the mean time.
>> >>
>> >> Can the following patch be modified to work with AAC too?
>> >
>> > I don't like the patch as it is, but you've helped me quite a bit to
>> > figure this out. :)
>> >
>> > lpc[0] = 1 isn't used so I'd like to remove it and start the useful
>> > coefficients from index 0. With this, the above-quoted loop can be
>> > rewritten to use part of compute_lpc_coefs():
>> >
>> > // tns_decode_coef
>> > for (m = 0; m < order; m++) {
>> >    float tmp;
>> >    lpc[m] = tns->coef[w][filt][m];
>> >    for (i = 0; i < m/2; i++) {
>> >        tmp = lpc[i];
>> >        lpc[i]     += lpc[m] * lpc[m-1-i];
>> >        lpc[m-1-i] += lpc[m] * tmp;
>> >    }
>> >    if(m & 1)
>> >        lpc[i]     += lpc[m] * lpc[i];
>> > }
>> >
>> > Doing it this way avoids the need for the b[] temporary array and
>> > produces identical output I think. You should be able to see clearly
>> > that this is the same as the main loops within compute_lpc_coefs().
>> > So, now the question is how to refactor this code to satisfy everyone.
>> >
>> > Michael - would you like a single generic macro like the one Vitor
>> > proposes? Or maybe two macros, one for this inner set of loops and one
>> > for the extra autocorrelation function return value normalisation to
>> > convert to autocorrelation coefficients and checks that are needed for
>> > the other implementations? Or something else?
>>
>> Actually, I think an inline function will be better. I'll have a look
>> a go at implementing this after I've eaten.
>
> thats exactly what i wanted to suggest, all these macros are ugly ...
> (the reason behind vitors macros was IIRC a compression loss with float
>  vs.double in relation to flac but i didnt had the time to investigate
>  what excatly was causing it or if there is a better solution ...)

Hmm. Will this cause a problem if I use a double temporary variable? I
think I've figured everything else out in my head. Though maybe
there's something I've missed, I'll find out shortly. :)

Rob